Combined-cycle power generation systems typically include gas turbines, steam turbines, generators, heat recovery steam generators (HRSG's) and controls, all optimized and matched to form an economical power plant. Single-shaft combined-cycle systems consist of one gas turbine, one steam turbine, one generator and one heat recovery steam generator with the gas turbine and steam turbine coupled to the single generator in a tandem arrangement on a single shaft. Multi-shaft systems have one or more gas turbines/generators and HRSG's that supply steam through a common header to a separate single steam turbine-generator. Multiple-pressure reheat steam cycles are typically applied in conjunction with gas turbines that have high exhaust gas temperature as a means of increasing the energy conversion efficiency of the bottoming steam cycle.
In a reheat combined-cycle system, steam is typically supplied back from the steam turbine to the heat recovery steam generator, where it is heated and returned to the steam turbine. This is not a particular problem with a single-shaft combined-cycle system with one steam turbine and one HRSG which receives heat from the gas turbine because the reheat steam is supplied directly from the one steam turbine to a single HRSG. Application of a reheat steam cycle to a multi-shaft combined cycle has typically been restricted to a configuration in which cold reheat steam is distributed to each of only two HRSG's for reheating and subsequent return to the steam turbine. This restriction is a result of the complexity, expense and operability problems that arise when more than two HRSG's are involved. That is to say, the distribution and recollection of reheat steam becomes greatly complicated in terms of piping, valves and other ancillary equipment. The reheat steam distribution problems are most severe in the case of repowering an existing nuclear steam turbine which, because of its size, may require ten or more gas turbines and HRSG's to supply steam up to its rated capacity. Thus, a highly complex system of piping, valves and controls with corresponding high expense would be necessary in order to take the steam from the high-pressure steam turbine exhaust, and supply it to ten or twelve different HRSG's and then return the steam to the steam turbine. Consequently, the full performance potential of the combined cycle has been compromised in large, multi-shaft configurations due to the practical necessity of using a non-reheat steam cycle.